Feat/todo management: Add GitLab to-do management tools

[bthall]

Summary

• Adds four MCP tools wrapping GitLab's GraphQL todo API: list_my_todos, mark_todo_done, mark_all_todos_done, restore_todo.
• The list tool introspects currentUser.todos at runtime and drops filters the self-hosted schema doesn't support, surfacing a _warning in the response (same fallback pattern as update_issue).
• Write tools inherit the existing four-step token resolution — GITLAB_READ_TOKEN is automatically rejected; GITLAB_TOKEN or per-call user credentials are required.

Changes

• src/gitlab-client.ts — four new methods (one GraphQL query + three mutations).
• src/tools.ts — four new tool definitions registered in searchTools and writeTools.
• README.md, CHANGELOG.md — documented.